Distributing liquid samples or reagents to a plurality of sample vessels in small quantities is an indispensable operation during experiment or analysis activities in the biochemical field, for example. The distribution is performed by sucking/discharging liquid through a distribution nozzle. The distribution nozzle is normally fitted with a disposable distribution tip. The distribution tip is replaced with a new one at any time whenever necessary.
A distribution tip is fitted to the distribution nozzle by inserting the lower end of the latter into a pipet-shaped distribution tip. The firmness of fitting depends solely on the elastic fastening force of a resin distribution tip surrounding the lower end portion of the distribution nozzle. Because of dimensional dispersions in the inner diameters of distribution tips, the fastening forces sometimes fall short of a level required to insure a normal fitting state. This invites a misfitted state with the distribution tip.
During a distribution operation, if a distribution head proceeds without having a distribution tip perfectly fitted thereto, a receiving plate will have a portion void of sample liquid. For assuring reliability of the test result, misfitting of a distribution tip should be avoided by all means. In a conventional apparatus, where a single distribution tip is used, the existence, or non-existence, of a distribution tip may be checked upon through an electrical conductivity test using a conductive material for the distribution tip. Or, it may be confirmed indirectly by watching a pressure during a suction operation. The above-described checking methods, however, are hardly applicable to a distribution head that has multiple distribution tips. The problem with the conventional automatic distribution apparatus is that it is difficult to detect a misfitted distribution tip with a high certainty level.